Aliphatic polycarbonates are biodegradable eco-friendly polymers. Poly(ethylene carbonate) and poly(propylene carbonate) as aliphatic polycarbonates can be prepared via alternating copolymerization of carbon dioxide as a monomer with the corresponding epoxide. The use of carbon dioxide is of great environmental value (Reaction 1). A catalyst with ultrahigh activity for carbon dioxide/epoxide copolymerization reaction was developed by the present inventors and is currently ready for commercialization under the trademark Green Pol (Korean Patent No. 10-0853358). The number of carbon atoms in the carbonate linking groups of aliphatic polycarbonates prepared via dioxide/epoxide copolymerization is limited to 2. Poly(ethylene carbonate) and poly(propylene carbonate) as representative aliphatic polycarbonates have limited physical properties, such as low glass transition temperatures of 40° C. and 20° C., respectively, and lack of crystallinity.

Aliphatic polycarbonates whose carbonate linkers each have three or more carbon atoms can be prepared via ring-opening polymerization of the corresponding cyclic compounds (Reaction 2). Such ring-opening polymerization has the advantages that no by-products are formed and final polymers have high molecular weights (e.g., weight average molecular weights of hundreds of thousands) (Pego A P, Grijpma D W and Feijen J, Polymer 2003, 44, 6495-6504); Yamamoto Y, Kaihara S, Toshima K and Matsumura S, Macromol Biosci 2009, 9, 968-978). However, the monomeric cyclic compounds are not easy to produce and their use is thus not suitable for the commercialization of aliphatic polycarbonates. That is, the trimethylene carbonate shown in Reaction 2 is currently sold at a price of about 158,000 won per 50 g by Aldrich and is thus unsuitable for use as a monomer for the preparation of general-purpose polymers. The (tetramethylene carbonate) dimer and (hexamethylene carbonate) dimer are not distributed in the market and are produced through complicated isolation and purification processes using enzymes. Accordingly, the use of the dimers is inappropriate for mass production of aliphatic polycarbonates on a commercial scale.
The most appropriate method for mass production of aliphatic polycarbonates whose carbonate linkers each has three or more carbon atoms is associated with the condensation of dimethyl carbonate or diethyl carbonate and various diols (Reaction 3). Dimethyl carbonate and diethyl carbonate are inexpensive compounds that have been produced from phosgene. Efforts have been made to develop processes for the production of dimethyl carbonate and diethyl carbonate using carbon monoxide or carbon dioxide instead of toxic phosgene. The use of environmentally friendly carbon dioxide is more advantageous. Dimethyl carbonate and diethyl carbonate produced by these processes are in practical use at present.
There are many reports in the literature on the preparation of aliphatic polycarbonates via the condensation reaction shown in Reaction 3. However, Reaction 3 for the preparation of aliphatic polycarbonates is slow and has a limitation in increasing the molecular weight of the final polymers. Due to difficulties in the preparation of high molecular weight polymers, oligomeric macrodiols whose molecular weight is several thousands and both ends are capped with —OH groups are currently produced and used for polyurethane production. It was reported that macrodiols having a molecular weight not higher than 2,000 can be produced by condensation of 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol with dimethyl carbonate (DMC) using a calcium catalyst (J. Appl. Polym. Sci. 2009, 111, 217-227). However, the overall reaction time is as long as 36 hours. After the reaction, the low molecular weight macrodiols are dissolved in acetone and the solid catalyst component is filtered off. According to a recent report, macrodiols having a low molecular weight on the order of 1,000 can be produced through a condensation reaction between 1,6-hexanediol and DMC using calcined MgAl hydrotalcites as solid bases (Ind. Eng. Chem. Res. 2008, 47, 2140-2145). The low molecular weight polymers are obtained in the form of oils and the residual catalysts are removed by filtration. In this case as well, the reaction time is very long (≧12 hours). Other reports are also found in the literature on the synthesis of macrodiols with a molecular weight on the order of several thousands and the production of polyurethane using the macrodiols (U.S. Patent Publication No. 2010/0292497; EP 302712; EP 1874846). The synthesis of the macrodiols usually requires a long reaction time of at least 10 hours and the use of a strongly basic catalyst. When a sodium alkoxide is used as the catalyst, the resulting polymer is dissolved in an organic solvent after the reaction and washed with water to remove the catalyst. In contrast, when the catalyst is a titanium alkoxide, no process is described for removal of the catalyst.
Efforts have also been made to prepare high molecular weight aliphatic polycarbonates. Sivaram et al. reported the preparation of aliphatic polycarbonates having weight average molecular weights of 6,000 to 25,000 by condensation of DMC with various diols (e.g., 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, and 1,4-bis(hydroxymethyl)cyclohexane) using 1,3-diphenoxytetra-n-butyldistannoxane as a catalyst (Polymer 1995, 36, 4851-4854). The overall reaction time is 11 hours and the reaction temperature is raised to 220° C. The reaction is carried out via a two-step process to increase the molecular weight of the polymers. After step 1, each reaction product is dissolved in methylene chloride and washed with water to remove unreacted diol compound. That is, as a strategy to increase the molecular weights of the polymers, water-insoluble oligomers end-capped with methyl carbonate are subjected to a condensation reaction while removing DMC in step 2. However, no effort to remove the catalyst is found in the report. U.S. Pat. No. 5,171,830 discloses a process for the preparation of aliphatic polycarbonates including condensing DMC with various diols using a tertiary amine or alkylammonium salt as a catalyst. According to a representative example of this patent, 1,4-butanediol is reacted with excess DMC at 150° C. for 8 hours to prepare mono- or bis(methyl carbonate) esters of 1,4-butanediol and a condensation reaction of the mono- or bis(methyl carbonate) esters is induced while removing volatiles under vacuum or reduced pressure at an elevated temperature up to 200° C. to increase the molecular weight of the polymer. However, the molecular weights of the polymers prepared by this process are only on the order of 2,400 and the end groups of the polymers are capped with methyl carbonate. For removal of the catalyst, the polymers are dissolved in chloroform and dropped into an alcoholic solvent to obtain precipitates. According to a recent report, an attempt has been made to synthesize aliphatic polycarbonates by condensation of various diols and DMC using 1-n-butyl-3-methylimidazolium-2-carboxylate (1 mol %) as a catalyst. However, the aliphatic polycarbonates have number average molecular weights not higher than 6,700 and their end groups are capped with methyl carbonate. For removal of the catalyst, the polymers are dissolved in THF and dropped into an alcoholic solvent to obtain precipitates (Polym. Chem. 2012, 3, 1475). Recently, Chuncheng Li et al. reported the preparation of a high weight average molecular weight aliphatic polycarbonate by condensation of DMC and 1,4-butanediol using a TiO2/SiO2/poly(vinyl pyrrolidone) mixture as a solid catalyst (Polym. Int. 2011, 60, 1060-1067; Journal of Macromolecular Science, Part A: Pure and Applied Chemistry 2011, 48, 583-594). The overall reaction time is about 10 hours. They took a strategy to increase the molecular weights of the polymer by preparing an oligomer end-capped with methyl carbonate in step 1 and inducing a condensation reaction of the oligomer while removing DMC in step 2. The creation of vacuum or reduced pressure at a high temperature of 200° C. is required to increase the molecular weight of the polymer. However, under these temperature and pressure conditions, a small amount of tetrahydrofuran (THF) is formed as a by-product. A thermally stable resin is obtained by dissolving the polymer in chloroform and precipitating the polymer in methanol (Polymer Degradation and Stability 2012, 97, 1589-1599).